In the past food and pharmaceutical grade phosphoric acid or alkali metal phosphates including, for example, trisodium phosphate, sodium tripolyphosphate, tetrasodium pyrophosphate and disodium phosphate were prepared by the thermal acid method described in Slack, Phosphoric Acid, V. 1, pp. 927-966, Marcel Dekker, Inc. (1968). The phosphoric acid and phosphates used in food, pharmaceuticals and in the processing of foods and pharmaceuticals must within stringent limits be substantially free from arsenic, fluorides, heavy metals and general contaminants referred to as insolubles.
The thermal process for producing food and pharmaceutical grade phosphoric acid produces extremely pure phosphoric acid, but requires extensive capital investment for equipment, a large input of energy and pollution control equipment. Until recently, the inability to confidently remove contaminants made electrothermal phosphoric acid prepared from elemental phosphorous the predominant source of pure phosphoric acid.
The so-called wet process employing phosphate rock and sulfuric acid is well known in the art and described in Waggamen Phosphoric Acid, Phosphates and Phosphatic Fertilizer, pp 174-209, Hafner Publishing Co. (2nd Ed., 1969). The wet process is also broad enough to include acids other than sulfuric, including nitric and hydrochloric acid. This is described in Slack, Phosphoric Acid, V. 1, Part 2, pp. 889-926, Marcel Dekker, Inc. (1968).
Many processes were proposed to remove the contaminants found in wet process phosphoric acid and substantially all the successful processes employ activated carbon. For example, see U.S. Pat. No. 3,993,733; U.S. Pat. No. 3,872,215; U.S. Pat. No. 3,993,736; U.S. Pat. No. 3,122,415; and British Pat. No. 1,442,919. Unfortunately, the phosphate rock impurities deactivate the porous activated carbon and because sufficient of these impurities are inorganic, thermal regeneration of the activated carbon is unsuitable because phosphate glasses fuse to the pores of the activated carbon. Since the use of activated carbon was the key to so many successful processes for upgrading wet process phosphoric acid, however, some means to increase the life of the too quickly exhausted activated carbon was necessary, since mere disposal after only limited use is clearly economically unsatisfactory. Attempts were made to backwash the activated carbon with poor results. Washing with caustic as in British Pat. No. 1,103,224 at first seemed a satisfactory solution, but the activated carbon never returns to more than 90% of the immediately previous cycles capacity, so after four regenerations the capacity is reduced to about 66 % of the virgin activated carbon and after ten regeneration cycles with caustic alone, the carbon is substantially exhausted.
Clearly, unless inordinate quantities of activated carbon are to be employed a more efficient regeneration means is required to not only save the capital cost and operating of renewing the activated carbon, but also the labor employed in replacement.
The process of this invention returns the activated carbon to a condition equivalent to its virgin capacity, it achieves the results required to make wet process phosphoric acid the equivalent of thermal elemental phosphoric acid, since processes using activated carbon now become commercially feasible.